1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of speed stages to be established by a selecting operation of a plurality of hydraulic engaging elements. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
As this kind of apparatus, there has hitherto been known one in which a hydraulic pressure of that hydraulic engaging element on disengaging side which is disengaged at the time of speed changing (also called a disengaging pressure) and a hydraulic pressure of that hydraulic engaging element on engaging side which is engaged at the time of speed changing (also called an engaging pressure) are electronically controlled by using solenoid proportional valves. An input and output speed ratio (rotational speed of an output shaft/rotational speed of an input shaft) of a transmission is detected, and a condition of progress (or development) in speed changing is discriminated from the input and output speed ratio to thereby control the disengaging pressure and the engaging pressure.
In this kind of apparatus, there is also known one in which the following arrangement is made. Namely, at the time of upshifting, in order to prevent the engine from racing, the engaging pressure is gradually increased while controlling the disengaging pressure such that the hydraulic engaging element on the disengaging side does not slip, i.e., such that the input and output speed ratio of the transmission lies within a predetermined engaging region which is set based on the gear ratio of the speed stage to be established by the engagement of the hydraulic engaging element on the disengaging side. When the input and output speed ratio has risen to a value above the engaging region as a result of decrease in the rotational speed of the input shaft due to simultaneous engagement of the hydraulic engaging element on the disengaging side and the hydraulic engaging element on the engaging side, a discrimination is made that the condition of speed changing has transferred from a torque phase to an inertia phase. The engaging pressure is then increased to a predetermined high pressure and also the disengaging pressure is decreased to a predetermined low pressure.
In the upshifting control of the above-described conventional example, if the rate of gradual increase in the engaging pressure in the torque phase is made large, the engaging force of the hydraulic engaging element on the engaging side at the time of transferring to the inertia phase becomes excessive, with the result that shocks are likely to occur. As a solution, the engaging pressure is increased relatively gradually (or slowly) and, as a consequence, it takes much time to transfer to the inertia phase, and the time required for the upshifting becomes long.
In view of the above point, the present invention has an object of providing a control apparatus for a hydraulically operated vehicular transmission in which the upshifting can be performed in a short time without giving rise to shocks.